Expose URLLIB connection pool KW

[romanzdk]

Add option to specify urllib kwargs. We would use it e.g. for max size.

[romanzdk]

Solves #1009 (comment)

[romanzdk]

@penelopus would you have a few minutes to check, please?

[justinpolygon]

@romanzdk I'll look at this today. I just want to make sure we're not going to break backwards compatibility or the way people are connecting today. From my tests is looks fairly safe but anything that touches the way we initialize the client I want to be careful of.

[romanzdk]

Hey @justinpolygon , have you had a chance to take a look at this, please? :)

[romanzdk]

kind reminder @justinpolygon